Numerous products in the market today are extruded thermoplastics. Such products include vinyl siding, picture frames, tubing, rain gutters and down spouts to name just a few. Thermoplastics such as polyvinyl chloride offer significant advantages in the market place because of their ease of use in forming the product, the finished product's durability and capacity to handle extremes of temperature as well as the ability to color and texturize the product to suit consumer demands. For purposes of example only, the issue of dimensional quality control will be discussed in the context of post-form extruding thermoplastics into a wide variety of vinyl siding styles, textures and colors. Dimensional quality control utilizing the method and apparatus described herein can be applied to a wide variety of products.
Products such as vinyl siding must be manufactured to exacting tolerances in order to withstand the extremes of temperature found in many regions of the country and throughout the world. In addition, the siding must be resistant to moisture, be able to resist damaging impacts from hail, rocks and other objects launched by lawnmowers and even balls and toys thrown by children. Vinyl siding that does not meet design specifications can be poor fitting and consequently aesthetically unappealing thereby leading to product recalls and significant consumer dissatisfaction. Vinyl siding with features that do not mate well because the siding fails to meet design specifications will result in a product that can be very difficult for the installer to attach to the building, or attach to the building without an unsightly fit.
Two commonly employed techniques for producing vinyl siding are profile extrusion and post-form extrusion. Profile extrusion utilizes an extrusion machine to heat powdered thermoplastic resin, typically poly-vinyl chloride, and under great pressure, forces it out of a die in the desired cross-section profile. Though very effective at producing high quality products with dimensional stability, it is a relatively slow process with feed rates in the range of 1.5 to 6 meters per minute (5 to 15 feet per minute). As with nearly all production lines, the speed of production is critical in order for the business to be profitable.
The technique that is most commonly employed for production of vinyl siding is post-form extrusion. With post-form extrusion, thermoplastic resin is heated to temperatures around 200° C. and forced under pressures of as much as 13.8 Kilopascals (2000 pounds per square inch) from an extrusion device. The extruded thermoplastic is then formed into a flat sheet and may undergo other processes such as embossing or cooling before being fed into a water cooled fixture that is commonly referred to as a calibrator. The calibrator serves to bend and twist the flat sheet into the desired profile as the flat sheet is pulled through the calibrator by the haul-off machine at the end of the production line.
The fixtures used to form the flat sheet into the final cross section are machined to very exacting tolerances with internal surfaces that are ground to a mirror finish and are typically plumbed with communicating passages for application of a vacuum source to pull the moving flat sheet against the upper surfaces of the calibrator. Pulling the sheet against the upper surface of the calibrator facilitates increased processing speed by reducing the prospect for jamming and binding of the thermoplastic material against the bottom surface of the calibrator as it moves at speeds of up to 50 meters per minute (165 feet per minute).
As the flat sheet moves into the calibrator it is shaped and formed into the desired cross section. Increasing demand for vinyl siding has prompted manufacturers to seek ways to economically increase their productivity. One way to increase productivity is to increase the haul off machine rate thereby increasing the rate at which resin must be fed into the extruder. Increasing the haul-off rate can have the unintended consequence of changing the dimension of the finished product. Different production lines utilizing nearly identical calibrators can also have different tensions placed on the sheet being pulled through the production equipment. Likewise, varying extrusion device temperature ranges and even varying resin mixtures particularly when coupled with different pull rates can have subtle, yet palpable, impacts on the dimensional integrity of the vinyl siding features.
Accordingly, a substantial need exists for a methodology that can be utilized to adjust the dimensions of features of the post-form extruded vinyl siding after the siding exits the calibrator and before the thermoplastic has had an opportunity to fully set in its final form. A further need exists for a low cost and functional apparatus that can simply and efficiently be utilized to adjust the dimensions of critical features of a post-form extruded product, such as vinyl siding after it exits the calibrator.